Conventional belt tensioning devices usually comprise a motor and a traction transmission means, one end of which is coupled to a belt buckle or to an end fitting of the safety belt.
Such a device is known from the DE 199 61 799 A1. An electromotive drive serves, in the case of an imminent accident to the vehicle, to bring a belt buckle from a normal position into a provisional safety position, so that an increased traction is exerted onto the safety belt band. This procedure is generally designated as pre-tensioning and only requires a fraction of the force necessary for the actual tensioning of the belt band. If an impact actually occurs, a separate pyrotechnically actuated tensioner drive takes over the emergency tensioning procedure, in which the belt buckle is moved into a final safety position. Otherwise, the belt buckle is returned into its normal position again by means of a compression spring.
A disadvantage in a pyrotechnically actuated tensioner drive is that it can only be used once. In addition, such a drive only provides the driving energy necessary for the tensioning of the belt band over an extremely short period of time. If, for any reason, the belt band can not be tensioned sufficiently during this period of time, there is no possibility for continuing the tensioning procedure.
The invention provides a device for tensioning a safety belt which can be used several times and provides a constant force for tensioning the belt band over a sufficiently long period of time.